1. Field of the Invention
The present invention relates to an offshore fish cage for farming fish, and more particularly to a structure for a net which forms such an offshore fish cage for farming fish.
2. Description of the Prior Art
One example of a fish cage set in the sea for farming fish is disclosed in "Fishery Civil Engineering", published by KOGYOJIJITSUSHINSHA on Dec. 1, 1979. FIG. 1 is a schematic view of prior art fish cages for farming fish. In FIG. 1, (a) shows schematically a square fish cage for farming fish, (b) a cylindrically shaped fish cage for farming fish, and (c) fish cages for farming fish which are moored. As clearly seen from FIG. 1, in conventional systems cages for farming fish are shaped in such a manner that net 13 is hung down from frame 12 floating on the surface of the water. Net 13 is supported by means of floats 11. The bottom of net 13 is pulled downward by the aid of sinkers 14 or an underwater frame 15. In these vessels, young fish are released and farmed. These fish farming vessels are connected to mooring floats 16. The mooring floats are in turn moored by means of mooring wires 17 to anchors 18 which have been sunk to the bottom of the sea.
Marine fish farming has hitherto been carried out by using square or cylindrically shaped fish cages for fish farming set in bays, where waves and tidal currents hardly affect the fish cages. Generally speaking, in such bay areas the sea is so calm, shallow and tideless that fish excretions, etc., easily accumulate. For this reason, because of the recent increase in the number of such fish farming cages, being used red tide has often occurred and many fish have become sick. This results in a reduced yield and a degraded quality of farmed fish. Furthermore, since in bay areas water temperature and salinity is much more changeable than in the ocean, growth of fish is impaired. In order to overcome these difficulties completely, fish farming in the ocean is preferable, where movement of seawater is strong, the depth is far greater, and water temperature and salinity is less changeable. However, the ocean waves and tidal currents are too strong and severe for conventionally structured fish farming cages to withstand.
Several attempts have been made to provide an acceptable offshore fish cage for farming fish. Two such types of fish cages are discussed below.